1. Field of the Invention
The present invention relates to a three-axis acceleration sensor, more particularly to technology for preventing faulty electrical contacts at the acceleration detection sites.
2. Description of the Related Art
Technology for detecting acceleration on three mutually orthogonal axes is described in, for example, Japanese Patent Application Publication No. 2004-198243. A piezoelectric acceleration sensor of the type disclosed in this Publication is shown schematically in FIGS. 1 and 2. FIG. 1 shows a plan view; FIG. 2 shows a sectional view through line I1-I2 in FIG. 1.
This acceleration sensor has a thin square first silicon layer 10, a thick square second silicon layer 20, and a bonding layer 30 by which the first and second silicon layers are joined. Substantially L-shaped openings 11 at the inside four corners of the first silicon layer 10 create a frame-shaped peripheral attachment section 12, a square mass attachment section 13 located centrally inside the peripheral attachment section 12, and four thin elongate beams 14 that link the mass attachment section to the peripheral attachment section. The four beams 14 are oriented in the x-axis and y-axis directions, which correspond to the lateral and longitudinal directions in the plane of the drawing sheet in FIG. 1. A pair of piezoelectric resistive elements 15-1, 15-2 are formed on the surface of each beam 14. The whole surface of the beams 14, including the piezoelectric resistive elements 15-1, 15-2, is covered with an interlayer dielectric film (not shown), on which metal wiring (also not shown) is formed. The metal wiring is electrically connected to the piezoelectric resistive elements 15-1, 15-2 through contact holes formed in the interlayer dielectric film.
The second silicon layer 20 comprises a peripheral frame 21 formed below the peripheral attachment section 12 in the first silicon substrate 10, surrounding a cavity 22 that extends in the vertical or z-axis direction clear through the second silicon layer 20 below the openings 11 and beams 14. A solid rectilinear mass 23 is formed below the mass attachment section 13, surrounded by the cavity 22. The height of the frame 21 in the z-axis direction exceeds the height of the mass 23. The upper surface of the frame 21 is bonded through the bonding layer 30 to the bottom surface of the peripheral attachment section 12, and the upper surface of the mass 23 is bonded through the bonding layer 30 to the bottom surface of the mass attachment section 13. The bottom surface of the frame 21 is bonded to a base 31.
The four beams 14 allow the mass 23 to sway or move in the x-axis, y-axis, and z-axis directions. When the sensor is accelerated, the mass 23 is displaced by a force proportional to the acceleration, the beams 14 bend, and the resulting strain changes the electrical resistance of the piezoelectric resistive elements 15-1, 15-2. The change is detected from signals routed through the piezoelectric resistive elements via the contact holes and the wiring. Suitable signal processing yields measurements of acceleration on each of the three axes.
When the beams 14 bend, the greatest stress occurs at the boundaries P1 between the peripheral attachment section 12 and the beams and the boundaries P2 between the mass attachment section 13 and the beams. Therefore, to maximize the sensitivity of the sensor, the piezoelectric resistive elements 15-1, 15-2 must be disposed near the boundaries P1, P2, where they will experience the greatest strain. At these locations, however, a large mechanical stress also acts on the interface between the wiring in the contact holes and the silicon surfaces of the piezoelectric resistive elements 15-1, 15-2, and can produce such unwanted effects as wire peeling and separation, which degrade the reliability and shorten the life of the sensor.
Given the trend toward smaller sensor form factors, the contact holes in future products can be expected to become smaller, so that the contact resistance will increase. Failure modes occurring within the contact holes (such as, for example, voids in metal wiring and silicon nodules or aggregates) will also become more likely to affect the contact resistance, and variations in contact resistance will become increasingly prevalent. Since the output current produced by a piezoelectric resistive element is very small, the contact resistance tolerance value is also small. Variations in contact resistance can therefore quickly push the resistance value above the tolerance level, so that the sensor does not perform as designed.
These problems have a serious effect on sensor life and reliability. In addition, the anticipated reduction of contact hole sizes and the attendant increased variation in contact resistance values will have a serious effect on future sensor performance.